The present invention relates to a method of determining a principal subject image of color originals based on color densities, and a method of determining exposures in color printing which is made in accordance with the principal subject image of color originals.
In making positive color prints on color paper from color originals such as color negatives, color positives or the like, it is usual to control the exposure or the proportion of the primary color (blue, green and red) components of printing light in accordance with the large area transmittance densities (hereinafter referred to as LATDs) of the primary colors of the color original from which color prints are made. This printing system is well known in the art as a LATD printing system.
It is also well known in the art that almost all ordinary color originals include a background besides a principal subject image.
Since, in the above-described conventional LATD printing system, the determination of exposures is independent of the primary color densities of the principal subject image of color originals, the obtained prints are not always satisfactory because of improper color balance and/or color density of the principal subject image thereof.
In order to solve the above-described problem in the LATD printing system, there has been proposed a method of color printing in which the image of a color original is displayed on a CRT screen of a color monitor and a point on a principal subject image such as the human face is specified by means of a light pen in order to measure the primary color densities of the point according to which the proportion of the primary color components of printing light are controlled. This method is disclosed in, for example, Japanese Patent Unexamined Publ. No. 58-9136.
In this method, however, there is a problem that, since only a part of the principal subject image of the color originals is specified to be measured, the color densities cannot be obtained accurately for the whole area of the principal subject image. This problem can be solved by specifying the boundary between the principal subject image and its background image by using a position specifying means such as a light pen. This solution is, however, undesirable and hardly practicable owing to a troublesome and time-consuming operation of specifying the boundary. As a matter of course, in order to determine the exposure or the proportion of the primary color components of printing light based on the color densities of a part of the principal subject image of color originals, it is necessary to obtain exact primary color densities. However, this is very difficult in practice because there are a large number of color originals of high contrasts and/or small principal subject images. In addition, the fact of disregarding the background ground when determining the exposure results in reproducing unfaithful prints.
There has also been disclosed in, for example Japanese Patent Publ. No. 55-29412, a method of color printing for solving the above-described problem in which a color original is divided into plural image sections, one of them being chosen as a principal subject image. The chosen image section is scanned at a number of points (picture elements) to obtain a plurality of color densities thereof, from which at least one kind of characteristic density value is calculated. Based on the characteristic density values thus obtained and the LATDs of the color original, the exposure or the primary color components of printing light is controlled.
In this method, it is also necessary to obtain exact characteristic density values. Nevertheless, because of the relatively extensive divided sections, there is often included in the chosen section a part of a background image besides the principal subject image as a result of a visual analysis of an operator. For example, when the principal subject matter of the human figure has been photographed with a back light, the chosen section of the color original sometimes contains therein parts of the human figure and sky. As a result, confusion arises as to which is the intended principal subject matter: the human figure or the sky, and as a result, wrong characteristic density values will be used to control the exposure in color printing. In addition, the use of only the characteristic density values makes it impossible either to recognize color originals whose scenes are photographed with a back light or to calculate the value of exposure to be corrected in accordance with the LATDs.
There has been disclosed in, for example, Japanese Patent Unexamined Publ. No. 59-164547 still another method of color printing in which, in order to calculate exposures, one of a plurality of different formulas previously provided is selected through a selection key according to differences in density between the principal subject and its background images of color originals, the difference being judged by an experienced visual observer. In this method, it is possible to control the exposure with a high accuracy depending on closely grouped scenes in which the color originals are classified. This classification of color original, which is effected through visual analysis by the operator, is attended by individual differences in visual analysis, uncertain judgments and the like, and the provision of the closely grouped scenes is not always efficient. Consequently, the method is, although basically highly efficient, useful with only several scenes.
There has also been disclosed in, for example, Japanese Patent Unexamined Publs. Nos. 52-156624 and 53-145620 a method of color printing in which a particular extent of color on negatives is defined corresponding to flesh color on positives (this particular extent of color is hereinafter referred to as flesh color). A color original is scanned to obtain the primary color densities for each of a number of points (picture elements) on the color original in order to automatically judge whether or not each picture element has its color falling within the limits of flesh color previously defined, so as to control the exposure based on the mean color densities of the picture elements of flesh color. According to this method, in case the color originals include walls or the like which have a color similar to the flesh color of the human body, it is apt to take the walls for a part of the human body and to confuse gray color with flesh color, this color confusion being attributable to changes on standing in the characteristics of the film and the illumination lamps, resulting quite often in incorrect exposure controls in color printing.